Paul, InDesign has no problem opening PageMaker files - I've opened a few since switching to InDesign over a year ago. InDesign is great - I'd never go back to good ole "RageMaker" - though I never had any problems with it, it seems downright primitive compared to InDesign. I never was a Quark user, though I own 4.x....and don't see myself forking over $$$ for the OS X upgrade, since I'm very happy with ID.
The conversion extension which Tacit mentioned is $141.55 for the download version(for Quark 4.x) or $189 for the download for the Q5 version and there is NO demo for either.(Markzware offers demos of lots of other things though). Unless you're planning on using Quark a lot, save the $141 and apply it to the purchase of ID.
InDesign can also open Quark files, BTW.